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If you had a blowout on the highway, or you’re just in the mood to switch things up a little, it’s time to get yourself a fresh set of tires. But you can’t just walk into your local automotive store and grab any tire of your choosing. It needs to match the tire size of your vehicle.
You don’t need to do any crazy measurements to figure this out. In fact, your existing tires already have all the information you need. You’ve probably seen a combination of letters and numbers on the sidewall of your wheels before. No, that’s not the wi-fi password -- it’s your tire size.
Figuring out what all that means is pretty much impossible if you don’t know what you’re looking for, but once you know what the numbers mean, you can read any tire like an open book.
Okay, so it’s time to get a new tire. The first thing a technician is going to ask you is the tire’s size, and you’ll be able to look like a know-it-all. First and foremost, go find your tire’s sidewall, which is pretty much where you’d expect: the side of the tire. Around the edge you’ll see a series of letters and numbers. From left to right, here’s what they all mean.
The first thing you’ll see on the left side of the sequence is a letter. This is the type of vehicle that this tire is meant for. This letter is either P-Metric or Euro-Metric, and with the former being most common. This means you’ll likely see the letter “P” right at the front, denoting that this tire is meant for a passenger vehicle. If it’s Euro-metric, you may not see a letter at all.
However, you may also see the letters “LT.” This stands for a light truck. Other abbreviations include “T” for temporary spares or “ST” for special trailer.
Knowing the tire type is important because LT tires generally require higher inflation pressures than passenger tires. You likely wouldn’t have a light truck wheel on your sedan, but it’s good to know just in case your dad makes you fill the pick-up with some air.
The number that proceeds this lettering is your tire’s width from sidewall to sidewall. It’s the diameter of the wheel from one end to another. It’s always measured in millimeters. So for example, if you see a measurement that reads P225, this tire is for a passenger vehicle with a width of 225 millimeters.
Note that sometimes the tire type is listed after the width. So the first number may look like 225P instead of P225. We wish there were more standardization here too, but it’s easy enough to figure out once you know the drill.
As we keep moving right, you’ll notice a slash mark. Directly after that is the tire’s aspect ratio, which is the height of the tire’s cross section to its width. Basically, it’s a measure from the center of the wheel rim to the top of the tread.
Event though it doesn’t come with the % symbol, this number is written as a percentage. You can figure out the aspect ratio by dividing a tire’s height off the rim by its width. If a tire has an aspect ratio of 65, it means the tire’s height is 60% of its total width.
You can then find the sidewall height by multiplying these two numbers together. In our example, if you convert 225 millimeters into inches, you get 8.86. Then multiply that by 60% (.6) to get a numerical value for the sidewall, which would be 5.32 inches.
A lower aspect ratio means that the tire has a shorter sidewall. These are typical in your sedans and sports cars that have more of a reliance on control and speed rather than off-road durability. Wheels with high-aspect ratios look more like a donut, with sidewalls that extend much further away from the rim.
After the aspect ratio, you’ll see another letter. This one indicates the type of internal construction that holds together your tire’s stability.
Chances are, this letter is an “R,” which stands for radial. These are the most common tires in the US, and it basically means that the internal ply cords are manufactured in a radial direction. Essentially, the cords run perpendicularly to the direction of rotation. However, you may still see a “D” or a “B” which stand for diagonal and bias ply, respectively.
So let’s do a little recap with our example: a tire that reads P225/60R is a passenger tire with a width of 225 millimeters, 60% sidewall ratio, and radial construction. We’re getting there!
Following the construction letter is another number that refers to the rim diameter. This is a measurement, in inches, of the size of the rim that this tire can be mounted. So if that number is 15, it means the tire is made for a rim with a 15 inch diameter.
Why is the tire width in millimeters but the rim diameter is in inches? We wish we knew.
Moving over to the right, you’ll be greeted by yet another number. This is the load index, which basically tells you how much weight the tire can support when fully inflated.
It’s confusing, though, because that number doesn’t actually tell you the number of pounds the tire can carry. Instead, the number corresponds to an exact load capacity in a tire index chart. Starting at one and going all the way to 150, it represents tires that can carry capacities of anywhere from 99 to 7385 pounds. We know… it’s confusing.
There are two load types: Standard Load and Extra Load. If it’s standard, there is no special marking. But Extra Loads are denoted by the letters XL at the end of the sequence. XL tires are reinforced to carry heavier loads than tires of the same size.
If a tire has a load index reading of 95, it can carry a maximum load of 1,521 pounds.
We’ve finally made it to the end. The last digit you should see in the sequence is the speed rating, which is indicated by another letter. Similar to the index number, this letter refers to a particular speed capability based on a standard manufacturer’s test.
The speed rating isn’t required by law, so there’s a chance it may not be listed. However, if you’re curious about your own car’s speed rating, you can look at some of the ratings here. Your passenger car will probably fall in the R or S ratings, equalling 106mph or 112mph maximum speeds respectively.
This doesn’t mean you should start driving that fast just because you can. Always follow the speed limit on roadways.
Okay, so let’s put it all together now that we’ve got everything in check. Our example tire reading looks like this: P225/60R15 95R. That means that our tire is for a passenger vehicle and has a width of 225 mm, a ratio of 60%, and it’s manufactured with a radial construction. The diameter of the rim is 15 inches and its load index is 95 (1,521 pounds), with a speed rating of R, or 106 mph.
Phew! We worked up a sweat figuring that out. But now you can see how the sizing sequence just looks a lot more complicated that it actually is. If you can read one tire, you can pretty much read them all.
Go treat yourself to an ice cold beer after getting through all of that. Deciphering a tire’s size is pretty difficult stuff. With that said, it can save you a ton of time in the long run, and your local automotive technician will thank you for your hard work.
But if you’re still a little unsure, it never hurts to get some help from the experts. Before dropping racks on your next set of rims, consider using Rent A Wheel to get yourself back on the road after getting some expert recommendations. From there, pay with cash or choose from one of our flexible payment plans.
With more options than you can count, we’ve got the right tire to suit your needs. And since you’ll probably be tired out after cracking the tire size code, we can deliver your new wheels right to your front door.